U.S. patent number 4,333,149 [Application Number 06/109,813] was granted by the patent office on 1982-06-01 for microprocessor-based state of charge gauge for secondary batteries.
This patent grant is currently assigned to General Electric Company. Invention is credited to William S. Passman, John S. Sicko, Dale F. Taylor.
United States Patent |
4,333,149 |
Taylor , et al. |
June 1, 1982 |
Microprocessor-based state of charge gauge for secondary
batteries
Abstract
A state of charge gauge for measuring the state of charge of
secondary batteries, such as the type employed in electric
vehicles, includes a microprocessor which, when supplied with data
varying in accordance with battery discharge current and battery
terminal voltage, determines battery resistance. Having determined
battery resistance which is a dynamically varying parameter
dependent on battery temperature and age, the microprocessor
computes the total battery charge capacity. Comparison of the
quantity of battery charge already depleted with the previously
computed total battery charge capacity yields an accurate
indication of remaining battery charge.
Inventors: |
Taylor; Dale F. (Schenectady,
NY), Sicko; John S. (Schenectady, NY), Passman; William
S. (Salem, MA) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
22329694 |
Appl.
No.: |
06/109,813 |
Filed: |
March 6, 1980 |
Current U.S.
Class: |
702/63; 324/433;
320/149 |
Current CPC
Class: |
G01R
31/379 (20190101); G01R 31/389 (20190101); G01R
31/3648 (20130101); G01R 31/3842 (20190101) |
Current International
Class: |
G01R
31/36 (20060101); G01N 027/42 (); G06F
015/56 () |
Field of
Search: |
;364/481,483
;324/432-434,76A,65R,62R,429,428 ;320/43,44,48 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Design of Primary and Secondary Cells; C. M. Shepherd; Journal of
Electrochemical Soc., Jul. 1965, vol. 112, pp. 657-664..
|
Primary Examiner: Wise; Edward J.
Attorney, Agent or Firm: Steinberg; William H. Davis, Jr.;
James C. Snyder; Marvin
Claims
What is claimed is:
1. A state of charge indicator for measuring the state of charge of
secondary batteries and for providing a visual indication of
remaining battery charge comprising:
a data circuit adapted to be coupled to a secondary battery under
load for providing first and second output signals proportional to
the magnitude of battery discharge current and battery terminal
voltage, respectively;
processing means coupled to said data circuit for computing battery
dynamic resistance in accordance with the magnitude of battery
discharge current and the magnitude of battery terminal voltage and
for computing total battery charge capacity in accordance with
battery dynamic resistance and battery charge already delivered and
determining the remaining battery charge by comparing total battery
charge capacity with battery charge already delivered; and
display apparatus coupled to said processor unit for providing a
visual indication of remaining battery charge determined by said
processor unit.
2. The invention according to claim 1 wherein said data circuit
comprises:
an analog to digital converter adapted to be coupled to said
battery under load and providing digital output data proportional
to the magnitude of battery discharge current and battery terminal
voltage; and
a multiplexer coupled to said analog to digital converter for time
multiplexing said analog to digital converter output data.
3. The invention according to claim 1 wherein said processing unit
comprises:
a first memory for storing a battery testing program
a second memory for storing a floating point arithmetic program
which is executed in conjunction with said battery testing program
stored in said first memory, and for storing a monitor program;
and
a central processor coupled to said first and said second memories
for executing said monitor program and, in accordance with said
monitor program, executing said battery testing program and said
floating point arithmetic program to provide, in accordance with
the magnitude of battery terminal voltage and battery discharge
current, a digital output signal representative of remaining
battery charge.
4. The invention according to claim 3 wherein said first memory
comprises a random access memory.
5. The invention according to claim 3 wherein said second memory
comprises a read-only memory.
6. The invention according to claim 3 wherein said central
processor comprises a microprocessor.
7. The invention according to claim 1 wherein said display
apparatus comprises:
a digital to analog converter coupled to said processing unit for
converting said processing unit output into an analog voltage;
and
meter means for visually indicating said digital to analog
converter output voltage.
8. A battery tester for discharging a secondary battery and for
providing a visual indication of remaining battery charge capacity
during battery discharge intervals, comprising:
discharge apparatus adapted to be coupled to a secondary battery
for discharging said battery in accordance with a discharge command
signal; and
a state of charge indicator which includes:
a data circuit coupled to said battery for providing first and
second output signals proportional to the magnitude of battery
discharge current and battery terminal voltage, respectfully;
processing means coupled to said data circuit and said discharge
apparatus for supplying said discharge apparatus with said
discharge command signal, and for computing battery dynamic
resistance in accordance with the magnitudes of battery discharge
current and battery terminal voltage and for computing total
battery charge capacity, in accordance with battery dynamic
resistance and battery charge already delivered by the battery and
determining the remaining battery charge capacity from the
difference between total battery charge and battery charge already
delivered; and
display apparatus coupled to said processing means for providing a
visual indication of battery charge determined by said processing
means.
9. A method for determining the state of charge of secondary
batteries under load comprising the steps of:
(1) sampling battery discharge current and battery terminal
voltage;
(2) computing battery dynamic resistance in accordance with the
magnitude of sampled battery terminal voltage and battery discharge
current;
(3) computing battery total charge capacity from the time discharge
begins from the dynamic resistance and the amount of charge the
battery has delivered from the start of discharge;
(4) comparing the battery total charge capacity with the amount of
charge the battery has delivered from the start of discharge to
produce the remaining battery charge; and
(5) visually displaying an indication of remaining battery
charge.
10. The method according to claim 9 further comprising the steps
of:
(6) repeating steps (1) and (2);
(7) recomputing total charge capacity from the time discharge
begins to update the calculated initial battery total charge
capacity from the dynamic resistance and the amount of charge the
battery has delivered from the start of discharge;
(8) repeating step (4) and (5);
(9) repeating steps (6-8) at least once.
11. The method according to claim 9 wherein said step of computing
battery dynamic resistance further comprises the steps of
determining the difference between a predetermined voltage and the
sampled battery voltage and dividing the result by the sampled
battery current.
Description
BACKGROUND OF THE INVENTION
This invention relates to apparatus for measuring the state of
charge of lead acid batteries, such as may be employed in an
electric vehicle, and for providing output data indicative of
battery charge.
Decreasing supplies of, and increasing prices for, refined
petroleum products such as diesel fuel and gasoline have prompted
increased interest in development of an electric vehicle suitable
for short distance (i.e. less than 125 miles) travel. Because a
sizable number of conventional internal combustion engine
automobiles and trucks are only used to traverse such short
distances, use of electric vehicles in their place could help
lessen domestic dependence on expensive imported oil, as the energy
required for electric vehicle battery charging could be supplied by
hydroelectric or nuclear power stations.
The maximum range of the electric vehicle is dependent on the total
charge capacity of vehicle batteries and battery charge depletion
per mile in exactly the same manner that the range of a
conventional internal combustion engine vehicle is dependent on
fuel tank capacity and fuel consumption per mile. However, unlike
conventional petroleum-fueled vehicles whose fuel tanks can quickly
be refilled in a matter of minutes, recharging of electric vehicle
batteries usually requires several hours. Therefore, an accurate
indication of battery charge capacity is required to apprise
electric vehicle user personnel of remaining battery charge so that
the electric vehicle is not driven a distance beyond that which
would permit safe return to a home base, or such other location
where battery charging can readily be accomplished. To simplify
electric vehicle operation, it would be desirable to display
remaining battery charge in an analog fashion much the same way
that the quantity of remaining fuel is displayed by conventional
internal combustion engine vehicle fuel gauge.
Traditional means for determining the state of charge of secondary
(i.e. rechargeable) batteries, such as the type used in electric
vehicles, have included current integrating devices such as the
electrochemical coulometer. The electrochemical coulometer
determines the total charge, that is ##EQU1## passing through a
shunt circuit, by depositing an amount of indicating material, such
as silver, at one side of an electrolysis cell proportional to the
amount of charge passed during a given interval. Resetting of the
electrochemical coulometer occurs during battery charging as
battery charge current carries indicator material to the opposite
side of the cell.
Electrochemical coulometers suffer from the disadvantage that the
indication of battery charge capacity they provide does not vary in
accordance with battery age or temperature. A "full charge"
indication by the electrochemical coulometer may be particularly
inaccurate at low battery temperatures, as battery charge capacity
decreases substantially as battery temperature decreases. Battery
charge capacity also decreases as battery age increases.
To remedy the disadvantage of such traditional means for
determining secondary battery charge capacity, the present
invention provides an indication of battery charge capacity in
accordance with battery resistance, a dynamically varying parameter
which is dependent on battery temperature and age.
BRIEF SUMMARY OF THE INVENTION
Briefly, in accordance with the preferred embodiment of the
invention, a state of charge gauge for measuring the state of
charge of secondary batteries, such as the lead acid type, and for
providing a visual indication of remaining battery charge comprises
a data circuit coupled to a secondary battery under load for
providing a first and a second output signal proportional to the
magnitude of battery discharge current and battery terminal
voltage, respectively. A processor unit, coupled to the data
circuit, computes battery dynamic resistance and generates a signal
indicative of total battery charge capacity. Comparison of total
battery charge capacity with the quantity of battery charge already
depleted yields an output signal, which is proportional to
remaining battery charge. A visual indication of remaining battery
charge is provided by a display apparatus, typically comprised of
an analog meter, in accordance with the processor unit output
signal.
It is an object of the present invention to provide apparatus for
measuring the charge capacity of secondary batteries and for
providing a visual indication of battery remaining charge.
It is another object of the present invention to provide apparatus
for measuring the charge capacity of secondary batteries and for
providing a visual indication of remaining battery charge which is
compensated for battery temperature and battery age.
BRIEF DESCRIPTION OF THE DRAWINGS
The features of the invention believed to be novel are set forth
with particularity in the appended claims. The invention itself,
however, both as to its organization and method of operation,
together with further objects and advantages thereof, may best be
understood by reference to the following description taken in
conjunction with the accompanying drawings in which:
FIG. 1 is a block diagram of a state of charge indicator according
to the present invention, as employed in measuring the state of
charge of secondary batteries within an electric vehicle;
FIG. 2 is a flow chart diagram of the monitor program executed by
the state of charge indicator of FIG. 1 during operation;
FIG. 3 is a block diagram of a battery tester employing the state
of charge indicator of FIG. 1; and
FIG. 4 is a timing diagram illustrating the operation of the
battery tester of FIG. 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 illustrates an electric vehicle drive system 10 comprising a
battery 12, configured of one or more lead acid cells. Battery 12
is coupled to a power converter 14 which supplies electrical energy
to a traction motor 16 connected thereto. The nature of traction
motor 16 determines the structure of power converter 14. Thus, if
traction motor 16 comprises a synchronous or induction alternating
current motor, power converter 14 would then be comprised of an
inverter.
A state of charge indicator 20, according to the present invention,
for on-board electric vehicle use is coupled to the positive and
negative terminals of battery 12 via conductors 22a and 22b,
respectively, and is coupled to a current sensor 24, coupled in
series with battery 12 and power converter 14, via conductor 26.
Charge indicator 20 provides a visual indication of the remaining
charge of battery 12 in accordance with battery terminal voltage
and discharge current and includes a processing unit 28, which,
when supplied with data from a data circuit 30 proportional to the
magnitude of battery discharge current and battery terminal
voltage, computes remaining battery charge and provides an output
signal indicative thereof to display apparatus 32.
Determination of remaining battery charge by processing unit 28 is
achieved by reliance on the relationship between battery terminal
voltage and discharge current determined empirically by C. M.
Shepherd in his paper "Design of Primary and Secondary Cells"
published in the Journal of the Electrochemical Society in July
1965 in Volume 112 at pages 657-664. Shepherd states that constant
current discharge data for secondary batteries, such as the lead
acid type, can be given fairly accurately by equation (1):
where
E=battery terminal voltage
E.sub.s =a fitted constant representative of a reference voltage
which is substantially close in magnitude to the open circuit
battery terminal voltage
Q=a fitted constant representative of a reference battery charge
capacity which is indicative of battery charge capacity at low
discharge rates
N=a fitted constant representative of internal battery
resistance
K=a fitted constant representative of the coefficient of battery
polarization
i=battery discharge current
t=time
Equation (1) can be rearranged to describe a dynamic resistance r
as seen by equation (2)
which resistance, for a battery in a given condition, that is, at a
given temperature and age, varies only as a degree of battery
discharge. Examination of equation (2) yields the conclusion that
resistance r is independent of the rate of battery discharge and
thus, is indicative of total battery charge capacity.
Equation (2) can itself be rearranged to yield
In practice, r quickly becomes very much larger than N shortly
after commencement of battery discharge. With r>>N shortly
after commencement of battery discharge, ##EQU2## yields a value
for Q, the total battery charge capacity. Thus, measurement of
battery discharge current and battery terminal voltage, and
integration of battery discharge current with respect to time,
allow calculation of total battery charge capacity for a given
battery temperature and age. By subtracting ##EQU3## the amount of
charge depleted from the total battery capacity Q during the
interval commencing from the inception of battery discharge until
the present, a value Q.sub.r representative of remaining battery
charge can be obtained.
At the inception of battery discharge, when the disparity between r
and N is not as great as during the latter stages of battery
discharge, some correction for Q may be required. Such correction
for Q may be obtained from successive values of r and q, where
##EQU4## Substituting the value of ##EQU5## set forth in equation
(4) into equation (3) yields
Upon examination of equation (5), it is evident that when
r>>N, as is the case for latter stages of battery discharge,
a plot of r vs rq will be linear with a slope of 1/Q. Knowledge of
this fact allows accurate correction of Q from 3 separate values of
r and q, such as r.sub.1, r.sub.2 and r.sub.3, respectively, and
q.sub.1, q.sub.2 and q.sub.3, respectively, as follows:
However, the actual total battery charge capacity Q is obtained
from a plot of r vs (r-N)q so that ##EQU6## and thus
Solving equation (9) for N and substituting the resulting value
back into equation (8) yields two expressions for Q one of which is
as follows: ##EQU7## If q.sub.3 -q.sub.2 =q.sub.2 -q.sub.1 then
or
where
To compute the remaining battery charge, Q.sub.r, that is the
difference between Q and ##EQU8## in accordance with equations (1)
through (13) above, processing unit 28 comprises a central
processor 34 which is typically configured of a microprocessor such
as the Model 8080A microprocessor manufactured by Intel
Corporation. Coupled to central processor 34 is a clock 35
configured to generate an interrupt every 1/60th of a second. A
bidirectional data bus 36 couples central processor 34 to a 2K read
only memory (ROM) 38 and to a 1K random access memory (RAM) 40.
Read only memory 38 contains two programs, a monitor program,
described more fully below, which schedules the acquisition of
battery discharge data and execution of a testing program which
computes remaining battery charge from battery discharge data, and
a floating point arithmetic program executed in conjunction with
the testing program. Random access memory 40 stores the testing
program prior to execution by central processor 34.
Data bus 36 also connects central processor 34 to data circuit 30,
which, in the presently illustrated embodiment, comprises a two
channel analog to digital converter 42, coupled at the first input
via conductors 22a and 22b, to battery 12 and coupled at the second
input to current sensor 24, via conductor 26. A multiplexer 44
time-multiplexes the digital battery discharge data provided by
analog to digital converter 42 prior to transmission to central
processor 34.
Display apparatus 32 is also coupled to central processor 34 via
data bus 36 and, in the presently illustrated embodiment, display
apparatus 34 comprises a digital to analog converter 48 for
converting digital output data generated by central processor 34
into an analog voltage which is supplied to a meter 50 configured
to display remaining battery charge in accordance with the
magnitude of output voltage supplied by digital to analog converter
48. Although not shown, display apparatus 32 could also be
configured of a data communications terminal or a digital display,
comprised of either light emitting diodes or liquid crystal display
cells coupled to suitable amplifier circuitry to drive such devices
in accordance with output data supplied by central processor
34.
FIG. 2 is a simplified flow chart diagram of the monitor program
contained within read only memory 38 which is executed during
battery charge indicator 20 operation. The monitor program consists
of a real time scheduler and an idle time scheduler. During
execution of the real time scheduler portion of the monitor
program, scheduling of data acquisition and scheduling of the
battery testing program and the floating point arithmetic program
execution is initiated, while during execution of the idle time
scheduler portion of the monitor program, performance of scheduled
tasks occurs.
At the inception of monitor program execution, program variables
are each initialized at zero and clock 35 is rendered operative to
generate an interrupt signal every 1/60 of a second. When the clock
interrupt signal magnitude t.sub.c is unequal to zero, as is the
case each time an interrupt occurs, battery discharge data is
obtained from battery 12 via data circuit 30, both shown in FIG. 1,
and is stored in RAM 40 memory. Execution of both the battery
testing program and the floating point arithmetic program to
compute battery resistance and remaining battery charge is then
scheduled. Once remaining battery charge is computed, display of
remaining battery charge is then scheduled. When display of
remaining battery charge is completed, re-execution of the monitor
program is commenced.
During the "idle time," that is the time between the completion of
execution of the real time scheduler portion of the monitor program
and the occurrence of a succeeding clock interrupt signal,
execution of the battery testing program and the floating point
arithmetic program is commenced and display of the value of
remaining battery charge previously computed during battery testing
program and floating point arithmetic program execution, is
accomplished. Each time computation of remaining battery charge, or
display of the computed value of remaining battery charge is
scheduled, the task is stored in memory. During the execution of
idle time scheduler portion of the monitor program, these tasks are
performed and execution of the real time scheduler portion of the
monitor program is resumed following the occurrence of a clock
interrupt signal.
The charge indicator apparatus of the present invention can also be
configured with a discharge apparatus to provide a stand alone
battery tester, as shown in FIG. 3 for discharging secondary
batteries and for profiling battery charge during battery discharge
intervals. The battery tester of FIG. 3 comprises a state of charge
indicator 20, including a data circuit 30, configured identically
to data circuit 30 of FIG. 1. Data circuit 30 is coupled to a lead
acid battery 112 via conductors 22a and 22b, and is coupled via
conductor 26 to a current sensor 114 which is coupled in series
with battery 112 and the serial combination of a load, shown as
resistance 115a, and relay the contacts 117aa of a relay 117a.
Coupled in parallel with resistance 115a and the contacts 117aa of
relay 117a is the serial combination of resistance 115b and the
contacts 117bb of relay 117b, the serial combination of resistance
115c and the contacts 117cc of relay 117c, and the serial
combination of resistance 115d and the contacts 117dd of relay
117d. Typically, resistances 115a-115d are each equal in ohmic
value, with the ohmic value of each being selected such that when
each of relays 117a-117d is activated or energized, 30 amperes of
battery current passes through each of resistances 115a-115d,
respectively.
Each of relays 117a-117d is coupled to a relay controller 120,
which is typically comprised of a solid state stepper relay or the
like. Relay controller 120 is coupled to processing unit 28 of
charge indicator 20, which processing unit configured identically
to processing unit 28 of FIG. 1. Processing unit 28 is also coupled
to data circuit 30. During operation, processing unit 28 supplies
relay controller 120 with a discharge command signal and, in
accordance therewith, relay controller 120 activates one or more of
relays 117a-117d to commence battery discharge. In accordance with
battery discharge data supplied thereto by data circuit 30,
processing unit 28 computes remaining battery charge which is
visually displayed on a display apparatus 32 coupled to the
processing unit. Typically, display apparatus 32 comprises a data
communications terminal, such as the General Electric Terminet.RTM.
data communications terminal.
Operation of the battery tester of FIG. 3 may best be understood by
reference to the battery tester timing diagram of FIG. 4. As
illustrated, the processing unit commences execution of the real
time scheduler portion of monitor program to command activation of
one or more relays once every second thereby initiating battery
discharge. A predetermined time interval after relay activation,
battery discharge data is scheduled to be sampled N times, where N
is greater than 2 e.g. 7 as illustrated. By waiting to sample
battery discharge data until after a predetermined time interval
has elapsed following relay activation, the occurrence of spikes or
notches in the sampled battery discharge current and terminal
voltage will be greatly reduced. Accuracy of the sampled battery
discharge data is increased by frequent repeated samplings, as such
frequent repeated samplings effect digital filtering of battery
discharge data, thereby reducing any error that may be attributable
to extraneous noise. After sampling of the battery discharge
current and battery terminal voltage, execution of the battery
testing program and the floating point arithmetic program is
commenced to compute remaining battery charge capacity, which is
thereafter visually displayed. The periods of idle time occur
during data sampling when the central processor does no work.
The foregoing describes a microprocessor based state of charge
indicator for providing a visual indication of remaining battery
charge in accordance with battery discharge current and battery
terminal voltage and which adjusts for battery temperature and
age.
While the invention has been particularly shown and described with
reference to several preferred embodiments thereof, it will be
understood by those skilled in the art that various changes in form
and detail may be made therein without departing from the true
spirit and scope of the invention as defined by the appended
claims.
* * * * *